Many users have some deficiencies when it comes to seeing. This doesn't always mean that they are blind. For some people it is enough when fonts are clear and the color scheme can be adjusted.
This is something every application should do in any case, so here is the list:

Follow the user interface guidelines! This will get you quite far.

Check that color scheme changes apply

Try switching to a dark color scheme and see that your application is still usable

Test changing the font size

Switch to different fonts and see that they apply

Increase the font size and make sure that the application layout still works

When you have problems seeing, the mouse is quite hard to use. The keyboard is a lot more reliable. Therefor it is important that applications can be used with the keyboard. For some users, using a mouse is difficult because of motor skill issues.
Making applications keyboard accessible benefits everyone, since it allows us to use shortcuts to use the applications more efficiently.

Try to operate your application with the TAB key

Make sure that the tab order is correct (or fix it in Qt Designer or the code)

Start your application and do a common task without using the mouse

Note where you had trouble and think about possible improvements in the UI or keyboard shortcuts that are missing

As a sighted person you will probably never fully understand how users use screen readers. You will most likely not become proficient and efficient in their use. Nonetheless there is a lot you can help to make applications accessible to screen reader users. We refer to screen readers and other assistive technology often as AT.

This section gives a quick intro what to look for when testing an application with a screen reader.

Once you have an application running with the screen reader:
Make sure Orca says something intelligible for all elements. When it reads a
gui element it should say the label and type, eg: "File, Menu" or "OK,
Button".
When you have a button that does not have a label, maybe because it shows a
picture only, that's something to fix. Try navigating the more troublesome
elements - comboboxes and lists and such. Trees need a bit of love in the
bridge still.

Once you found a bug it's usually quite easy to fix. This section explains how to go about improving applications.

For many things there are usually easy fixes involving no advanced programming skills but just fixing some details.

For this tutorial we assume that you are dealing with a QWidget that is seen by the AT but does for example give not enough information.

There are two important properties that every QWidget has: an "Accessible Name" and an "Accessible Description".
The name is short, for example the label on a button. It should always
be short. The description on the other hand is the more verbose "this button
does foo and then bar". Qt will try hard to return something for the name. In case of a button, usually the text on the button is returned. But if your button has text that makes only sense when seeing the arrangement of buttons, or only has an image as label, you need to help the AT read the button.
If you don't, it will only say the type of the widget, "Button" is not a very helpful information.

Fire up Qt designer if the app uses .ui files. You'll find the properties and
can type the name/description right in. After saving the file, rebuild and install the application. You are done, submit a patch to fix the ui file.

If the widget is created in the code, just need to find where.
Once you found the widget, usually where it's created, add some code to it:

button->setAccessibleName(i18n("Open"));
button->setAccessibleDescription(i18n("Opens a file dialog to select a new foo"));

Send your patch.

Sometimes you also want to override the label for a different reason. One of my test apps was the calculator example from Qt. It has a memory recall button labelled "MR". Orca will insist on this being the "Mister" button, unless told otherwise.

For some widgets the above is not enough. You will have to create QAccessibleInterfaces for widgets that you create yourself.
For example Kate has an interface for its text editing area.
Sometimes you need to inherit QAccessibleTextInterface or QAccessibleValueInterface in order to make the widgets properly accessible.
Refer to the Qt documentation how to do this.

For Qt Quick with Qt 4.x there is no solution for accessibility.
For Qt 5, refer to the documentation on how to create accessible QML applications. The concepts are generally the same as for QWidget based applications.